Sometimes you might get into a situation where you owe a customer money. For example if they mistakenly overpay their account or the combination of orders, returns, credits means the customer ends up with a balance you own them. Depending on your policy you may or may not pay this balance back. If you do pay it back here is a quick walk through.
PS. If you want to have a look at an alternative approach to paying a customer their balance you can have a look at this post as an example of paying through the AR payment journal. https://organicax.com/2015/06/17/paying-a-customer-their-balance-second-option/